The present invention relates to the screw rotors applied to a screw machine, and to the screw machine such as a dry vacuum pump etc. using the screw rotors.
Conventionally, as a pump or a compressor which can afford a high-speed, long-time continuous operation, there is known a positive-displacement screw machine having a pair of screw rotors within its housing.
In such a kind of screw machine, e.g., the screw machine used as a dry vacuum pump, the male and female screw rotors in reverse screw relation with each other are arranged in parallel and meshed with each other so as to be spaced an infinitesimally small clearance apart, and, between the both rotors and the housing surrounding the rotors, there are formed the operation chambers comparted by the meshing portions of the rotors. Also, the screw machine is arranged such that the male and female screw rotors are rotated in synchronism, with the male and female screw rotors intermeshed in substantially a noncontact state, thus causing the volume of the operation chambers to increase on a suction side and to decrease on an exhaust side.
Also, in the vacuum pump, generally, the two phases of essential performance, i.e., the ultimate pressure and exhaust velocity thereof, are highly required. In the screw machine such as a dry vacuum pump, the meshing engagement of the male and female screw rotors arranged in parallel, as well as the clearance between the both rotors and the housing, exerts a great influence on any phases of the performance. Therefore, in such a screw machine, the clearance between the male and female screw rotors and the clearance between the both rotors and the housing are made small to the utmost, thereby seeking to improve the performance.
Further, the screw machine has some types such as a Lysholm type, a square threaded type (with a Quinby-shaped (square-shaped) tooth profile), and a spiraxial type (with a spiraxial screw tooth profile formed by combining an epitrochoid with an Archimedean spiral curve). In the Lysholm type, the one whose rotors have four threads or more each with the female rotor increased by one thread relative to the male rotor is in frequent use. In the square threaded type and the spiraxial type, the one in which the male and female rotors have one thread each is in frequent use.
In the case of the square threaded type or the spiraxial type, in the transverse cross section perpendicular to the rotation axis, the position of center of gravity thereof is heavily displaced from the rotation center. Hence, in order to strike a couple balance, it is necessary to form large cavities by means of as cast, etc., for opening cavities on the end faces of each of the screw rotors, thus causing the manufacturing process to be complex.
Further, the screw machine takes such a rotor form that, at the meshing portions of the male and female screw rotors, there occurs a difference of relative circumferential speed between the both rotors. Hence, it happens in some cases that the both screw rotors having a small clearance at the meshing portions undergo thermal expansion due to the high-speed, long-time continuous operation under a heavy load, etc., so that the both rotors are slidingly contacted, thereby causing seizure between the male and female screw rotors. Consequently, there is a problem that the meshing clearance between the rotors must be ensured even at the sacrifice of the pump performance to some extent so that such seizure between the rotors due to the thermal expansion may not occur.
Accordingly, the invention aims at reducing the meshing clearance between the screw rotors to improve the performance, and additionally an object thereof is to provide the screw machine capable of effectively preventing the seizure between the rotors even under a long-time high-speed continuous operation.
In order to solve the aforesaid problem, the invention is characterized in that, in the screw rotors which are each provided, around the rotation axis, with the screw tooth having a spiral addendum surface portion and the deddendum surface portion forming a spiral groove between the addendum surface portions, and are used as a pair of male and female in reverse screw relation with each other, between the addendum surface portion and the deddendum surface portion of the screw tooth, there is provided the pitch circumference portion which forms a predetermined angle range of circular arc having a definite radius on the optional transverse cross section perpendicular to the rotation axis.
Since the pitch circumference portion is provided, on the cross section perpendicular to the rotation axis, the position of center of gravity is never heavily displaced from the rotation center, and there is no need to form complex cavities by means of as cast, etc., thereby enabling reduction in the manufacturing costs.
In each of the screw rotors, the offset of the position of center of gravity from the rotation center becomes smaller as compared with the spiraxial type of screw rotor and the square threaded type of screw rotor both having the equivalent exhaust sectional area and rotor diameter. Hence, since the lead number is made integral to position the position of center of gravity on the rotation center axis, there is no need to form the complex cavities by means of as cast etc. for striking a couple balance.
By arranging such that the pitch circumference portion is formed in a band shape in the radial location substantially at the midpoint between the addendum surface portion and the deddendum surface portion, the tooth profiles of the respective rotors can be formed in common so as to facilitate the processing, and also the required sealing performance at the meshing portions of the screw rotors can be exerted by the pitch circumference portion having a definite width.
In the invention, it is preferable that the radius of the pitch circumference portion is set such that, when one of the pair of male and female rotors is meshed with the opposing rotor, of the meshing clearances between the male and female screw teeth, the clearance between the pitch circumference portions is smaller than the clearances between the other portions. In this case, when the male and female screw rotors are meshed with each other, of the meshing clearances between the male and female screw teeth, the clearance between the pitch circumference portions is smaller than the clearances between the other portions. Hence, the meshing clearance between the rotors becomes the smallest between the pitch circumference portions. Thereby, however, when the screw rotors undergo thermal expansion, the pitch circumference portions with a definite radius are initially abutted against each other into a rolling contact, and thus seizure is difficult to occur as compared with the conventional machine in which the tooth portions of the both rotors are most apt to be slidingly contacted.
Further, since the meshing clearance per se between the rotors can be maintained the smallest between the pitch circumference portions, the efficiency can be increased.
The screw machine according to the invention is characterized in that, with the screw rotors arranged as the male and female rotors meshed with each other, the both rotors are housed in parallel within the housing forming a suction port and an exhaust port, so as to be meshed with each other in a noncontact meshing engagement, and that, between the housing and the both rotors, there are formed a plurality of operation chambers which are transferred in the axial direction of the rotation axis by rotation of the screw rotors, and have volume increased in the transfer section communicating with the suction port, while decreased in the transfer section communicating with the exhaust port.
The present disclosure relates to the subject matter contained in Japanese patent application No. 2000-72893 (filed on Mar. 15, 2000), which is expressly incorporated herein by reference in its entirety.